PROJECT SUMMARY Frailty impacts up to 60% of older adults and is a leading cause of dependency among older adults. Frail older adults experience physical problems such as balance difficulties, weakness, decreased endurance, and reduced walking speed that increase risk for falls, hospitalizations, institutionalization, and death. Physical activity has multi-system health benefits and is the most recommended frailty management intervention, but guidelines regarding the specific intensity of physical activity remain unclear. High intensity physical activity is an established and safe therapeutic approach in other populations, but it is unclear as to the extent to which high intensity activity can reduce or reverse frailty. This cluster-randomized study will compare a high intensity walking (HIW) intervention to a self-selected, casual speed walking (CSW) intervention implemented within retirement communities for pre-frail and frail older adults. We will randomize 10 retirement communities with 20 participants at each site (200 total participants) to either a 4-month HIW or CSW intervention. All participants will receive 48, individually supervised overground walking sessions occurring within their retirement community. At the beginning and end of the study, we will measure participants? frailty, mobility, physical functioning, balance, and total physical activity measured via an ActivPal accelerometer worn for a 1-week observation period to compare which treatment strategy, HIW or CSW, worked better to reduce frailty (aim #1). We will use the SHARE-Frailty Instrument to evaluate frailty as both a categorical (e.g. non-frail, pre-frail, or frail) and a continuous outcome. This approach will allow us to determine how participants move between frailty categories as well as within frailty categories in response to intervention. In aim #2, we will determine the effect of walking intensity on mobility, physical functioning, balance, and total physical activity. We hypothesize that HIW participants will show decreased frailty and improved mobility, physical functioning, balance, and physical activity at 4 months. Currently, the optimal physical activity guidelines for older adults with frailty are uncertain. Results from this study will provide important knowledge to inform activity guidelines for older adults with frailty and information on a transformative approach to reducing frailty, improving function, and increasing physical activity among a growing segment of the older adults population.